Heat pipe regenerator for gas turbine engines



Feb. 25, 1969 M. F. PRAVDA ETAL 3,429,122

HEAT PIPE REGENERATOR FOR GAS TURBINE ENGINES f O t e e h s all In [I 6 6 9 l 7 v o N d e l 1. F

"TIL? INVENTORS MILTON F PRAVDA WILLIAM J. LEVEDAHL BY 5%... W, M,z; M

ATTORNEYS M. F. PRAVDA ETAL 3,429,122 HEAT PIPE REGENERATOR FOR GAS TURBINE ENGINES Sheet g of 2 FIGS 40 1M P 1, A 81160 3 R 0 N E V N ATTORNEYS Feb. 25, 1969 Filed Nov. 7, 1966 124 I26 I28 I30 3,429,122. HEAT PEPE REGENERATGR FOR GAS TURBINE ENGHQES Milton F. Pravda and William J. Levedahl, Baltimore, Md., assignors to Martin-Marietta Corporation, New York, N.Y., a corporation of Maryland Filed Nov. 7, 1966, Ser. No. 592,549 US. Cl. 60-39.51 Int. Cl. F02c 7/10; F02g 1/00; F2811 15/00 Claims AESTRACT OF THE DISCLOSURE This invention relates to the application of heat pipe principles to a gas turbine engine and more particularly, to a highly effective regenerative heat exchanger for use therewith.

A heat pipe, in its simplest form, comprises a container, normally metallic, employing on the inner surface thereof a capillary structure which is essentially saturated with a vaporizable liquid. The heat pipe acts to transfer heat, almost isothermally, from one point on the external surface to any other point by a vaporization-condensation cycle. In operation, a heat pipe consists essentially of four regions, each serving one primary function:

(1) The evaporator which transfers heat to the inflowing liquid, thereby vaporizing it.

(2) The vapor channel which permits vapor to flow from the evaporator to the condenser.

(3) The condenser which condenses the vapor by removing heat from it.

(4) The capillary liquid transport section in which the liquid condensate flows back to the evaporator.

The heat pipe, therefore, functions as a reflux condenser or evaporating-condensing device which uses the capillary or wick section to return condensed liquid from the condenser to the evaporator, replacing the liquid pump or gravity-induced natural circulation flow or centrifugal force in a rotary machine as is normally utilized in conventional systems. The heat pipe is a totally enclosed, simple, mechanically static device which can transport large quantities of heat over sizable distances essentially isothermally, and is not dependent upon gravity.

The efficiency and economy of either an open-cycle or close-cycle gas turbine power system is crucially dependent on the fraction of the energy contained in the exhaust gas which may be effectively employed to heat the compressed gas between the compressor(s) and the turbine(s). The efficiency of Rankine-cycle, boiling-condensing turbine power plants is also improved by utilizing heat from partially expanded working fluid extracted between turbine stages to heat the working fluid somewhere between the feed water pump and the superheater. The overall economy of any of these systems will increase as the cost (and thus the size) of the heat exchanger decreases, as the temperature differences between the two fluids in the heat exchanger decreases, as the pressure drop of the fluids through the heat exchanger decreases and as any leakage of fluid from one side of the heat exchanger to the other is decreased. A desirable de- 3,429,122 Patented Feb. 25, 1969 crease in any of the above characteristics generally comes at the expense of an increase in one or more of the others.

In most applications of gas turbine engines other than for ultrasonic or hypersonic aircraft, the overall efficiency of a gas turbine power plant is the highest when the exhaust gases are at the lowest possible temperature after expansion to their final pressure. High efliciency is also obtained when a temperature at entry into the turbine is at the maximum compatible with the materials limitations.

Thus, the exhaust gas must be as cool as possible when the turbine inlet is as hot as possible. This condition can be achieved by removing heat from the exhaut gases and transferring it to the gas leaving the compressor prior to combustion. Maximum efficiency can be obtained with relatively cool compressor-exit gases, and correspondingly, cool exhaust gases by applying low pressure ratios and/or compressor intercooling. Clearly, counterflow heat exchange is highly beneficial since the compressor-exit temperature can be approached closely by the exhaust temperature.

In an attempt to provide regenerative heat exchange, counterflow shell and tube regenerators, as well as rotating drum and rotating ceramic disc generators, have been employed in large industrial gas turbines as well as those for small automotive applications. While conventional shell and tube heat exchangers achieve the desired high efliciencies, they require bulky and complicated equipment to transport the exhaust gases and compressorexit gases into the same unit. Further, the pressure drops tend to be large if reasonably compact systems are utilized. On the other hand, rotating recuperators tend to decrease the space problem, since high surface areas may be built into relatively small drums or discs, and the surfaces are heated by the exhaust gases and then rotated into the compressor-exit stream which is then heated. The latter systems, while relatively compact, are not leak-tight and cause some carry-over intermixing of the two gas streams with corresponding pressure losses. They are also unlikely to closely approach the effective ness of a true counterflow heat exchanger. Further, the problem of diverting the compressor-exit and exhaust gases to flow in adjacent areas remains.

It is, therefore, a principal object of this invention to provide a highly effective regenerative heat exchanger for a gas turbine engine which is highly effective, thermally efiicient, completely static, requires minimum maintenance, is simple and relatively inexpensive to manufacture.

It is a further object of this invention to provide an improved regenerative heat exchanger for a gas turbine engine which will effectively transfer heat isothermally and which is normally immune to either gravitational or acceleration effects.

It is a further object of this invention to provide an improved, highly effective, regenerative 'heat exchanger for a gas turbine engine which provides ideal counterflow heat exchange without diversion of flow of either the exhaust or compressor-exit gases.

It is a further object of this invention to provide an improved, highly effective, regenerative heat exchanger for a gas turbine engine which greatly reduces both the size and weight of the regenerative heat exchange apparatus, which requires extremely low volume of working fluid, and in which the heat transfer is limited principally by airflow velocities exterior of the container holding the heat transfer working fluid.

It is a further object of this invention to provide an improved, highly eflicient regenerative heat exchanger for a gas turbine engine which may be readily incorporated in either terrestrial or aircraft applications.

Other objects of this invention will be pointed out in the following detailed description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of this invention and the best mode which has been contemplated of applying that principle.

In the drawings:

FIGURE 1 is a top plan view, partially schematic, of a gas turbine engine employing the regenerative heat exchanger of the present invention;

FIGURE 2 is a rear sectional view of the embodiment of FIGURE 1 taken about lines 2-2;

FIGURE 3 is a rear sectional view of the embodiment shown in FIGURE 1 taken about lines 33;

FIGURE 4 is a perspective sectional view of the parallel heat pipes forming the basic components of the regenerative heat exchanger of the present invention;

FIGURE 4a is an enlarged perspective view of a section of the regenerative heat exchanger of FIGURE 4 showing the formation of the capillary liquid transport means;

FIGURE 5 is a plan view of an alternate form of a gas turbine engine employing the heat pipe regenerative heat exchanger of the present invention;

FIGURE 6 is a rear sectional view of a portion of the apparatus shown in FIGURE 5 taken about lines 56;

FIGURE 7 is a plan view of yet another gas turbine engine showing an alternate embodiment of the heat pipe regenerative heat exchanger as applied to a gas turbine engine having split exhaust ducts;

FIGURE 8 is a rear sectional view, in section, of the gas turbine engine shown in FIGURE 7 taken about lines 88; and

FIGURE 9 is a side elevational view, in section, of a portion of the gas turbine engine shown in FIGURE 7 taken about lines 99.

In general, the invention is directed to an improved regenerative heat exchange system for a gas turbine engine which includes compressor, combustion chamber and turbine section. The improvement comprises heat pipe means for effecting isothermal heat transfer between the exhaust gases exiting from the turbine section and the compressed air passing from the compressor section to the combustion chamber. In a simplified form, duct means are provided for coupling the compressor section to the combustion chamber and for directing exhaust gases from the gas turbine section. The heat pipe means comprises multiple series of tubular heat pipes forming spaced, stacked planar arrays, having an intermediate transport section parallel to an exterior of the turbine element with right angle projecting sections terminating within respective first and second ducts. The second duct means may be disposed parallel to and in line with the first duet means with its straight heat pipe sections isothermally connecting the first and second duct means and passing through a common separating wall. The combustion products exhausting frm the turbine section of the gas turbine engine may be separated and reversely turned passing through multiple sections of the second duct means with the apparatus employing multiple heat pipe assemblies coupling the separate sections of the second duct means to the common compressor exit duct.

The present invention is directed to the application of heat pipe principles to a regenerative heat exchanger used in conventional in-line or reversely directed exhaust duct, gas turbine engines utilizing regenerative counterflow heat exchange principles to effectively reduce the temperature of the exhaust gases while heating the compressed gas from the compressor section prior to its entrance in to the combustion chamber. The invention consists essentially of a number of heat pipes, one part of each beat pipe being in the exhaust gas stream exiting from the turbine, and another part of each pipe being positioned in the gas stream exiting from the compressor.

Referring to FIGURE 1, there is shown somewhat schematically an in-line gas turbine engine 10 having three main, axially aligned sections, a forward compressor section 12 at the intake end 14 of the engine, a centrally located combustion chamber 16, and a turbine section 18. Positioned between the compressor section 12 and the combustion chamber 16 is a compressor-exit or duct section 20 which leads into the combustion chamber 16. The combustion products are discharged from the gas turbine at relatively high temperature and pres sure into a turbine exhaust section, indicated by dotted line 22, immediately to the rear of the turbine section 18. A series of spaced, parallel heat pipe assemblies 24, 26, 28, 30, 32 and 34 are carried by the gas turbine engine having ends positioned in the path of the com-pressed air exiting from compressor 12 and passing through duct 2t prior to entering combustion chamber section 16. The flow of compressed air from the compressor section through duct 20 is indicated by arrow 36. The other ends of the heat pipe assemblies are positioned within the exhaust gas duct section 22 in thermal contact with the exhaust gases exiting from the gas turbine section, as indicated by arrow 38.

Reference to FIGURE 1 shows the right-hand heat pipe assembly 34 which consists essentially of a stacked planar array of individual heat pipes 40, 42, 44, 46, 48 and 50. Each heat pipe is generally U-shaped having a forward end disposed within duct 20 and a rearward end disposed in duct 22. For instance, heat pipe 40 has a forward end 40 extending interiorly of cylindrical duct 20, in the path of the incoming high velocity air from cornpressor 12. Within exhaust duct 22, heat pipe 40 has a terminal end 40 which is disposed in the path of the high velocity, high temperature exhaust combustion products passing from turbine section 18. The arrangement is identical for the remaining heat pipes of each heat pipe assembly. For instance, heat pipe 50 includes a forward terminal or tip end 50' which is disposed within duct 20, as indicated best in FIGURE 2, and a rearward or tip end 50" which is positioned in duct 22 in the path of the discharging products of combustion from turbine 18. The portion of the heat pipe which is exterior of the turbine acts as a fluid transport section between the tip sections in the respective compressor duct sections 20 and turbine exhaust section 22. With respective to heat pipe 40, the intermediate section of the heat pipe 40", therefore, acts as a capillary transport and vapor transport section between the evaporator section 40 and the condenser section 40' of this particular heat pipe. Section 50 of heat pipe 50 performs the same transport function for both the liquid and gaseous and vapor phases of the working fluid carried thereby, in conventional heat pipe fashion, although its transport path is much shorter than the transport path 40 of heat pipe 40.

Each heat pipe, as further indicated in FIGURES 4 and 4a, is further provided with capillary flow means along the internal surfaces, completely along its length, the heat pipes being hollow, and therefore provided with a central void or vapor channel. For instance, heat pipe 40 is provided with capillary flow means, indicated by the dotted lines at 52 and a central void region 54. The construction of each heat pipe may be varied as long as it constitutes an enclosed chamber having evaporator and condenser surfaces, a vapor channel, capillary liquid transport means and a vaporizable fluid carried thereby.

In this regard, reference to FIGURES 4 and 4a shows one form which the heat pipes may take. Essentially, the stacked heat pipes 40, 42, 44 and 46 are in abutting contact, oriented in a common plane and consist principally of an outer tubular casing member 56 and an inner tubular member 58. The spaced tubular members 58 and 56, therefore, form a capillary flow channel 52 therebetween. The central void region 54 acts as a vapor channel, the vapor moving counterflow to the liquid carried within the capillary channel 52. The arrangement for each of the heat pipes is identical, as well as the direction of flow for the respective liquid and vapor phases.

Reference to FIGURE 4a shows in detail a preferred heat pipe configuration which includes suitable perforations in the inner tubular member. The inner tubular member 58 is spaced only slightly from the outer tubular member 56. The intervening space 52 which forms the capillary flow passage and defined by the generally parallel surfaces formed by the inner wall of tubular member 56 and the outer wall of tubular member 58 may be appropriately maintained by randomly punching perforations 60 along the wall surfaces of tubular member 58. In forming the perforations, the tool actually causes annular projections 62 to be formed outwardly of the plane of the wall member 58 which may readily be used to space the inner tubular member 58 with respect to outer tubular member 56. Further, in forming the projections 62, irregular slits 64 are created which allow ready fluid communication between the central vapor channel 54 and the capillary channel 52. Thus, any vapor which would tend be captured in the liquid phase carried by the capillary flow path 52 may escape inwardly to the central vapor channel 54 while liquid condensing along the inner surface of tubular member 58 may easily move into the liquid phase capillary carried by capillary channel 52.

The configurations of the heat pipe heat exchangers may be widely varied depending principally upon the configuration of the jet engine itself. For systems such as the axial flow aircraft engine of the embodiment of FIG- URE 1, the series of heat pipes are of identical, constant, rectangular cross-section but of different lengths. As a tube exhaust exists from the exhaust pipe or duct 22, it may be readily twisted bent to form along the length of the engine past the turbine and combustion chamber sections 18 and 16, respectively, and bent and twisted again at the point of entry into the compressor exit section or duct 20. As indicated, all the pipes in the same series are bent and twisted in the same way and then bundled together to form a tight structure. The individual pipes of the spaced pipe assemblies may be separated by thin layers of thermal insulation, if desired. Similarly, in some applications it will be beneficial to provide thermal insulation around the exterior of the entire heat pipe system exposed to the surrounding environment during normal system operation. Several series of pipe assemblies are preferably used in parallel. T o avoid ill effects from vertical accelerations, the pipes may all be oriented in an essentially horizontal plane or all exhaust sections might be below the corresponding compressor inlet sections. For small diameter engines, the sets of heat pipes may be readily symmetric.

It will be obvious to those skilled in the art that the working fluid of the heat pipe should have a sufliciently low melting point to prevent system freeze-up, a relatively high heat of vaporization to elfect efficient heat transfer and a safe vapor pressure at temperatures approaching that within the exhaust nozzle. Sodium and potassium are exemplary of working fluid materials which are satisfactory for use in most systems.

Turning to FIGURE 5, there is shown a second embodiment of the present invention as applied to a gas turbine engine having a different gas flow path. In the embodiment of FIGURE 5, the gas turbine is provided with a compressor section 112, a compressor-exit or duct section 120, a combustion chamber 116 and a turbine section 118 in the same in-line fashion of the embodiment of FIGURE 1. The exhaust duct or channel 122 is provided with a reverse trim portion 170 opening up into an enlarged cross-sectional portion 172 which is parallel to, abuts the compressor duct section and is coplanar therewith. The exhaust duct may be provided with an exhaust discharge opening 174 at the forward end of the gas turbine assembly at right angles to the axis of the compressor at its intake area 114. In this type of system, which readily permits complete freedom of selection of gas flow paths, the most simple type of heat pipe heat exchanger can be used. A series of heat pipe assemblies 124, 126, 128 and 130 extend between exhaust duct section 172 and compressor-exit section 120 separated by common wall member 176. Each of the heat pipe assemblies constitutes many straight, series heat pipes all of identical length and cross-section. For instance, in FIG- URE 5, the heat pipe assembly 139 includes abutting heat pipes 140, 142, 144, 146, 148, 150, 152, 156, 158, 160, 162 and 164. In like manner to the previous embodiment, the evaporator section 164' of heat pipe 164 is positioned in exhaust duct section 172 in the path of the exiting high speed, high temperature products of combustion as they emerge from the gas turbine section 118. The condenser section 164 of the heat pipe is positioned within the compressor-exit section or duct 120 in the path of the high velocity compressed air prior to entering combustion chamber section 116 of the gas turbine. In order to reduce the pressure drop in both compressor duct sections 120 and exhaust passage section 172, the heat pipes may have rounded leading and trailing adges. The dotted lines within the heat pipes (FIGURE 6) denote the capillary liquid transport means essential to proper heat pipe operation. The capillary liquid transport means of the heat pipes of this embodiment may be identical to that shown in FIG- URES 4 and 4a or, for instance, may comprise grooved internal surfaces of a single tubular member or integral spaced fins carried on the inner surfaces of the same tabular heat pipe casing or container. Vapor channels are provided centrally of the heat pipe and a vaporizable liquid, such as sodium, water, etc., is carried therein. The operation of the heat pipe is identical to that of the FIG- URE 1 embodiment.

It may be desirable to split the exhaust flow from the gas turbine and direct the same through a reversal using dual parallel heat pipe regenerative heat exchangers, each occupying one half of the compressor-exit area between the compressor and the combustion chamber sections of a gas turbine.

Reference to FIGURE 7 discloses yet another embodiment of the present invention involving a gas turbine engine 210 having compressor section 212, compressor-exit section or duct 220, in-line combustion chamber 216 and in-line turbine section 218. The exhaust duct indicated generally at 222 is split to form double, reversely turned duct sections 272 and 292 which open up into heat exchanger duct sections 273 and 293, respectively. The exhaust gases pass outwardly through exhaust openings 274 and 294 in opposite directions at right angles to the axis of the compressor 212 and its associated inlet 214. In like manner to the previous embodiment, multiple series of spaced heat pipe heat exchange assemblies 230 and 330, respectively, isothermally couple the compressor compressed air being discharged from compressor 212 to the exhaust gases passing through exhaust duct sections 273 and 293, respectively.

Reference to FIGURES 8 and 9 shows the manner in which the heat pipe heat exchanger assemblies are positioned within respective duct sections of the gas turbine engine. The ends of the heat pipe elements remote from the central axis of the gas turbine are positioned in the respective gas turbine exhaust ducts 273 and 293, in the path of the exhaust gas and therefore perform an evaporator function for the heat exchanger. The other ends of the same pipes are positioned within a common compressor-exit duct 220 and spaced slightly from each other. The inner ends of the heat pipes, therefore, perform a condenser function giving up heat to the compressed air. In both the duct sections 273 and 293, as well as the central compressor-exit duct 220, the leading and trailing edges of the heat pipe assemblies are preferably tapered at 296 (FIGURE 9) to reduce pressure losses. Each of the multiple series of heat pipes 330 comprises abutting pipes which may be preferably separated from each other by thin thermal insulation layers. In all other respects, the heat pipe assemblies, the individual heat pipes and their method of operation are identical to the previously described embodiments. The overall function is to isothermally transfer heat from the exhaust gases as they leave the gas turbine section to the incoming compressed air from the compressor prior to the entrance of the compressed air into the combustion chamber of the gas turbine engine.

The use of heat pipe regenerators can, in principle, provide ideal counterflow heat exchange without diversion of the flow of either exhaust or compressor-exit gases. The only losses are the pressure drops required to achieve any desired temperature difference between the exhaust and compressor-exit temperatures, these pressure drops can be made arbitrarily small by enlarging the heat transfer surfaces. Since heat transport within the heat pipe is accomplished by carrying latent heat of vaporization from the exhaust to the compressor-exit, relatively low flows of heat pipe working fluid are required and the size and weight of the heat pipes can be maintained relatively small. Heat transfer at the surfaces of the heat pipes will be limited by airflow velocities outside and not by the internal heat pipe evaporation, condensation or transport limitations. The heat pipes have been described as being tubular in construction and the word tubular is intended to cover heat pipes of rectangular or circular cross-section, as well as such unobvious configurations as ovoids, triangles, etc. As indicated, FIG- URE l, for example, is schematic in nature and naturally the system design will accommodate thermal expansion of its various components.

Maximum efliciency for a regenerative system without compressor intercoolers occurs at low compressor-pure ratios, as low as 4 to 1, when the turbine inlet temperature is at its highest permissible steady state value. If this maximum efficiency condition is at the normal steady state power level, such as normal cruise conditions for a jet, fan jet or turboprop engine, or normal highway power level for an automotive turbine or a normal rated power level for a central stationary power plant, then provisions must be made to permit high power to be drawn during take-off, climb or maneuvering an aircraft, for acceleration or hill climbing in an automotive engine, or transient overloads in a central station power plant.

Higher power levels imply increased airflow and higher compressor and turbine speeds, thus higher pressure ratios. If these changes are to be achieved with little or no increase in the turbine-inlet temperature, then the fraction of the total combustion heat removed from the exhaust and transferred to the compressor-exit gas must decrease. At steady state high power, this effect will naturally exist, since the exhaust temperature will decrease and the compressor-exit temperature will increase. In order to accentuate this effect and in order to permit a rapid transient to the higher power level without severe overtemperature at the turbine inlet, it is desirable to reduce exhaust back-pressure and reduce cooling of the exhaust gases. Both may be accomplished simultaneously by opening the exhaust nozzle in such a way as to permit exhaust gases to by-pass the heat pipes. Alternatively, by reference to FIGURE 1, a rather simplified arrangement is shown which has the attendant advantage that the heat pipe evaporator sections present in the gas turbine exhaust duct are physically removed and thus offer no impedance to the passage of exhaust gases. In such an arrangement, the forward or condenser ends of the heat pipes which are positioned within duct sections 20 intermediate of the compressor section 12 and the combustion chamber 116 are fixed thereto, while the evaporator sections of the heat pipe are freely positioned within exhaust duct section 22 much in the manner of a cantilever. Means, diagrammatically illustrated in FIG. 1 as consisting of a piston 400 pivotally connected between the combustion chamber 16 and the heat pipe assemblies 24, 26, 28, 30, 32 and 34 (not shown), may effect flexing of the individual heat pipe elements from the full line to the dotted line position. The evaporator ends of the heat pipe are then completely removed from the exhaust duct section 22 out of the path of the exhaust gases. In the dotted line position, the heat pipes cannot provide a back-pressure for the high speed gases and cooling of the exhaust gases is virtually eliminated. In respect to proper operation of the engine, it should be noted that the injection of additional fuel for power increase and the opening of exhaust bypass or movement of the heat pipes to the dotted line position should be properly sequenced.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in the form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A regenerative heat exchange system for a gas turbine engine including compressor, combustion chamber and turbine sections, the improvement comprising: a series of tubular heat pipes including condenser and evaporator sections for effecting substantially isothermal heat transfer between the exhaust gases exiting from the turbine section and the compressed air passing from the compressor section to the combustion chamber, means for positioning said evaporator section in the path of the exhaust gases and said condenser section in the path of the compressed air passing from said compressor section to said combustion chamber and means for selectively moving said evaporator section out of the path of said combustion products.

2. A regenerative heat exchange system for a gas turbine engine including compressor, combustion chamber and turbine sections, the improvement comprising: a first duct for fluid coupling said compressor section to said combustion chamber, a second duct coupled to said gas turbine on the exhaust side thereof, heat pipe means comprising at least one series of tubular heat pipes containing working fluid forming a stacked planar array and having a first projecting section terminating within said first duct and fixedly positioned therein and a second projecting section terminating within said second duct and freely positioned therein, whereby substanitally isothermal heat transfer is etfected between the exhaust gases exiting from said turbine section and the compressed air passing from said compressor section to said combustion chamber, and means for moving said second projecting section out of said second duct to terminate regenerative heat exchange and reduce back-pressure within said second duct.

3. A regenerative heat exchange system for a gas turbine engine including compressor, combustion chamber and turbine sections, the improvement comprising: a first duct for fluid coupling said compressor section to said combustion chamber, a second duct coupled to said gas turbine on the exhaust side thereof, heat pipe means comprising at least one series of tubular heat pipes containing working fluid forming a stacked planar array and having projecting sections terminating within respective first and second ducts, whereby substantially isothermal heat transfer is effected rbet-ween exhaust gases exiting from said turbine section and the compressed air passing from said compressor section to said combustion chamber, each said tubular heat pipe comprising inner and outer tubular members spaced slightly from each other and forming capillary flow passages therebetween.

4. The heat exchange system as claimed in claim 3 wherein the inner tubular member is randomly perforated to allow condensed liquid to move into the capillary flow passage and to allow vapor to pass toward a centrally located vapor channel.

5. A regenerative heat exchange system for a gas turbine engine including compressor, combustion chamber, and turbine sections, the improvement comprising: heat pipe means for elfecting substantially isothermal heat transfer between the exhaust gases exiting from said turbine section and the compressed air passing from said compressor section to said combustion chamber, said heat pipe means including inner and outer tubular members spaced slightly from each other and forming capillary flow passages therebetween.

References Cited UNITED STATES PATENTS Chausson 165-105 XR Cousins 60-3951 McCormick 60-3951 Beam 60-3951 US Cl. X.R. 

